A 62-year-old Indian woman undergoes total gastrectomy with D2 lymphadenectomy for a 6 cm proximal gastric adenocarcinoma involving the gastro-oesophageal junction. The specimen shows T4a N3 disease with 14 positive lymph nodes out of 28 resected. The proximal margin is 3 cm and distal margin is 8 cm, both negative for tumour. Postoperatively, she tolerates oral diet well. At 6 weeks post-op, she complains of severe postprandial bloating, early satiety, and loose stools. What is the most likely cause of her symptoms, and what is the first-line management?